A technology shown in FIG. 7 is known as a rotation angle detecting device which detects a wide range of rotation angles by use of a magnet and a magnetic sensor.
This rotation angle detecting device comprises a magnet 2 formed in the shape of a disc, a first magnetic sensor 4, and a second magnetic sensor 5. The center of the peripheral circle of the magnet 2 is coaxial with a rotational axis. In the magnet 2, a magnetic flux generation section is oriented in one of radial directions vertical to the rotational axis, and a magnetic flux absorption section is oriented in the other radial direction. The first magnetic sensor 4 which is disposed in the bottom face of the outer edge of the magnet 2 generates an output in accordance with magnetic flux radiated from the magnet 2. The second magnetic sensor 5 is disposed in the bottom face of the outer edge of the magnet 2 so as to be positioned at 90 degrees with respect to the first magnetic sensor 4 in a rotational direction. The second magnetic sensor 5 generates an output in accordance with magnetic flux radiated from the magnet 2.
When the rotational axis 1 rotates, the first and second magnetic sensors 4 and 5 generate sine waveforms with a phase difference of 90 degrees. In other words, when one output is in a sine curve, the other output is in a cosine curve.
By carrying out angle calculation with the two outputs, it is possible to detect the rotation angle of a rotary member over 360 degrees (refer to, for example, Japanese Patent Laid-Open Publication No. 2003-75108).
In the rotation angle detecting device according to the conventional example 1, the first and second magnetic sensors 4 and 5 are disposed in the vicinity of the periphery of the magnet 2. In the rotation angle detecting device, however, lines of magnetic flux generated by the magnet 2 are not uniform in a certain direction, and the lines of magnetic flux are oriented from the outer edge of the magnet 2 to the radial direction. Thus, when there is a slight misalignment (assembly error) between the first and second magnetic sensors 4 and 5, the vector components of magnetic flux applied to the magnetically sensitive surfaces of the first and second magnetic sensors 4 and 5 vary, so that the outputs of the first and second magnetic sensors 4 and 5 vary. Accordingly, precision in the detection of an angle becomes worse.
A rotation angle detecting device shown in FIG. 8 is known as a technology for preventing the foregoing problem.
In the rotation angle detecting device, a magnetic flux concentrator 3 made of a magnetic material is disposed in the vicinity of first and second magnetic sensors 4 and 5. The magnetic flux concentrator 3 regulates lines of magnetic flux applied from a magnet 2 to the first and second magnetic sensors 4 and 5, so that the vector components of magnetic flux applied to the magnetically sensitive surfaces of the first and second magnetic sensors 4 and 5 are prevented from varying (refer to, for example, Japanese Patent Laid-Open Publication No. 2002-71381).
In the rotation angle detecting device according to the conventional example 2, however, both of the first and second magnetic sensors 4 and 5 are fixed on a fixed member. A residual magnetic force on the magnetic flux concentrator 3 varies between the first and latter halves of the rotation (the forward rotation and the reverse rotation) of the magnet 2, and hence hysteresis occurs in the output characteristics of the first and second magnetic sensors 4 and 5. Thus, as shown in a solid line E2 (an output deviation amount in the first and latter halves of the first magnetic sensor 4) and a solid line F2 (an output deviation amount in the first and latter halves of the second magnetic sensor 5), the voltage outputs of the first and second magnetic sensors 4 and 5 largely differ between the first and latter halves of the rotation. When the voltage outputs of the first and second magnetic sensors 4 and 5 differ between the first and latter halves of the rotation, as described above, a deviation amount in a detection angle becomes large as shown by a solid line G2 (a deviation amount in a rotation angle) in FIG. 9B.
Namely, in the rotation angle detecting device according to the conventional example 2, the hysteresis makes precision in the detection of an angle worse.